A star coupler is a signal router commonly utilized in both microwave and optical frequency ranges. In the microwave context, star couplers are used to focus fields emanating from a phased array of antennas into a narrow beam, as described in, for example, N. Amitay et al., "Theory and Analysis of Phased Array Antennas," New York: Wiley, 1972. In the optical context, star couplers are useful in applications such as combining the outputs from laser arrays and multiplexing wavelengths in wavelength grating routers. For example, M. G. Young et al., "A 16.times.1 Wavelength Division Multiplexer with Integrated Distributed Bragg Reflector Lasers and Electroabsorption Modulators," IEEE Photonics Technology Letters, Vol. 5, pp. 908-10, August 1993, which is incorporated by reference herein, describes the use of star couplers to combine and focus light from an array of lasers. U.S. Pat. No. 5,002,350, which issued Mar. 26, 1991 to C. Dragone and is incorporated by reference herein, describes the use of star couplers to collect light from an array of waveguide arms in a wavelength grating router.
FIG. 1 shows a conventional optical star coupler 10 of the type described in greater detail in U.S. Pat. No. 4,904,042, which issued Feb. 27, 1990 to C. Dragone and is incorporated by reference herein. The star coupler 10 includes a high-index free space region 12 which is bounded by left and right circular arcs 14 and 16, respectively. The free space region 12 is typically formed as a slab of low-loss dielectric or other material. A number of input waveguide arms 18.sub.C, 18.sub.L.sup.i and 18.sub.U.sup.i are aligned in an array along the left arc 14, each pointing to the center of the right arc 16. A number of output waveguide arms 2.sub.C, 2.sub.L.sup.i and 20.sub.U.sup.i are aligned in an array along the right arc 16, each pointing to the center of the left arc 14. The input and output waveguide arms 18 and 20 each have a width w. The spacing a between the center lines of adjacent waveguide arms is referred to as the period of the coupler 10. In the conventional coupler 10, there is generally only one period, or unit cell, for each of the input or output waveguides. The radius of curvature R of the star coupler 10 is the distance between the centers of the left arc 14 and the right arc 16. A normalized angle .OMEGA. is defined such that a first Brillouin zone of the star coupler 10 extends from .OMEGA.=-0.5 to .OMEGA.=0.5 as indicated by the dashed lines in FIG. 1. Ideally, all of the optical signal light radiating from the star coupler 10 should be focused uniformly in the first Brillouin zone, and no stray light should exist outside that zone. In practice, substantial amounts of light fall outside the first Brillouin zone, such that the efficiency of a laser array or wavelength grating router incorporating star coupler 10 is significantly reduced. Moreover, the output radiation pattern in the first Brillouin zone of conventional star coupler 10 is not distributed evenly but is instead peaked in its center.
A number of techniques are known for increasing the efficiency of the conventional star coupler 10. One such technique, described in greater detail in C. Dragone, "Efficient N.times.N Star Couplers Using Fourier Optics," IEEE Journal of Lightwave Technology, Vol. 7, No. 3, pp. 479-489, March 1989, C. Dragone, "Efficiency of a Periodic Array with Nearly Ideal Element Pattern," IEEE Photonics Technology Letters, Vol. 1, No. 8, pp. 238-240, August 1989, C. Dragone, "Optimum Design of a Planar Array of Tapered Waveguides," Journal of the Optical Society of America, Vol. 7, No. 11, pp. 2081-2093, November 1990, U.S. Pat. 5,039,993 issued Aug. 13, 1991 to C. Dragone, and C. Dragone, "Optimal Finite-Aperture Filters with Maximum Efficiency," Journal of the Optical Society of America, Vol. 9, No. 11, pp. 2048-2054, November 1992, all of which are incorporated by reference herein, involves tapering the width of the waveguide arms using a gradual adiabatic taper, such that the waveguides arms are widest near the free space region 12. In a gradual adiabatic taper, most of the electric field remains in the fundamental or dominant eigenmode of a given waveguide arm, and very little power is transferred to other modes. This reduces the transition losses between the waveguide arms and the free space region, such that more optical signal light reaches the first Brillouin zone.
Another technique for improving the efficiency of the conventional star coupler 10 is described in C. van Dam et al., "Loss Reduction for Phased-Array Demultiplexers Using a Double Etch Technique," Integrated Photonics Research, Boston, Mass., pp. 52-55, Apr. 29-May 2, 1996, which is incorporated by reference herein. This technique involves adding a second high-index layer over the waveguide arms of the coupler, such that a vertical taper is produced. Like the above-described gradual adiabatic taper, this vertical taper approach reduces the transition losses between the waveguide arms and the free space region, such that more optical signal light reaches the first Brillouin zone. The vertical taper acts as a gradual adiabatic taper in the sense that the fields in the waveguide arms remain substantially in the fundamental eigenmode. Although the vertical taper is based on the same principles as the above-described gradual taper, vertical tapers are generally difficult to fabricate and therefore utilize an abrupt transition instead of a gradual transition. Nonetheless, star couplers using either the gradual taper or vertical taper approaches typically exhibit a multi-channel output radiation pattern which is peaked in its center, and therefore may not provide the optimal radiation pattern required in a given application.
It is therefore apparent that a need exists for improved techniques for shaping the output radiation pattern of a star coupler, without undermining the efficiency of the coupler, such that the radiation pattern can be optimized for a given signal routing application.